base-linux: reflect SIGCHLD as Cpu_session signal

With this patch, core responds to SIGCHLD signals of terminating Genode
processes by reflecting these events as exceptions to the CPU session
interface. This way, Genode processes become able to respond to
terminating Genode child processes.
base-linux: Fix delegation of invalid capabilities

This patch uses a local_name of -1 to encode an invalid capability.
core: Introduce default CPU exception handler

The CPU session interfaces comes with the ability to install an
exception handler per thread. This patch enhances the feature with the
provision of a default signal handler that is used if no thread-specific
handler is installed. The default signal handler can be set by
specifying an invalid thread capability and a valid signal context
capability.

Furthermore, this patch relaxes the requirement of the order of the
calls of 'exception_handler' and 'set_pager'. Originally, the exception
handler could be installed not before setting a pager. Now, we remember
the installed exception handler in the 'Cpu_thread' and propagate to to
the platform thread at a later time.